to Earthquake Resistant Design
The basic analysis paradigm that
Demand < Capacity
depends on having a structure, a realistic analytical model, an accurate computer program, & well defined loading. On the other hand, the design paradigm consists of finding a structure that:
Many analysis tools exist, but the structural engineer really needs design tools to come up with a structure that will fulfill the above objectives.
The Building codes are useful design tools - they define the "standard of care" - an important legal and professional concept. However, the focus is on the minimum standards needed for the protection of life safety. The mission of a building code is to
Difficulties with Using Codes
Professional liabilitiy concerns are increasing - are the simplfied code provisions adequate for complicated projects? In addition, current codes provide little guidance on how to achieve improved performance of new structures. A prescriptive format is used, rather than a performance-based approach. Building owners want better performance, but the code doesn't give engineers any methods for providing it.
The problem is not simply limited to a lack of performance-based criteria. Building codes do not treat many important subjects such as:
In addition, uneven performance of code-compliant buildings has been noted in recent earthquakes. Some buildings performed very well, while the performance of others was inadequate. Examples of inadequate performance in recent earthquakes include:
As a result, many building owners and public officials are unhappy with performance of code-compliant structures.
There is much activity to improve codes in order to rectify the inadequacies described above. Improvements are being made on several fronts simultaneously - improvements to existing national codes, as well as the development of new performance based approaches and guidelines for existing buildings. Improvements to national codes include:
New approaches which provide an emphasis on improved performance and reliability, as well as rational guidlines for the upgrade of existing buildings include:
The current situation is in flux! The new codes are in development, but aren't ready for use yet. In fact, they will probably take more than a decade to stabilize and become consistent. And the revised national codes may not fully define the state of practice in a particular area. Local groups are likely to write special provisions to accommodate local conditions. In addition, the developments do not all have the same performance objectives: